The invention relates to a method of fabricating optical fibers. In the method, layers of glass are deposited on the inner wall of a glass tube heated to a temperature between 1100.degree. and 1300.degree. C. A reactive gas mixture at a pressure between 1 and 30 hPa is passed through the glass tube, while a plasma formed in the tube is moved back and forth between two reversal points. The gas reacts to form the glass layers.
After glass layers corresponding to the intended structure of the optical fiber have been deposited, the glass tube is collapsed to produce a solid preform. Optical fibers are drawn from the preform.
In this method, "glass tube" is understood to be a substrate or supporting tube consisting of doped or undoped amorphous silica (fused silica or quartz glass) either made synthetically or made by melting silica crystals.
With the above-described method, both graded index fibers and stepped index fibers can be fabricated. Quantities of glass are deposited to correspond to the required fiber structure. The fabrication of optical fibers or optical waveguides by this method is known, for example, from U.S. Pat. Nos. Re 30,635 and 4,314,833. This fabrication process is known in the art as the "nonisothermal PCVD method", the P standing for Plasma and CVD for Chemical Vapor Deposition. In this method, glass layers are deposited directly from the gas phase onto the inner wall of the glass tube (a heterogeneous reaction). This avoids the formation of glass soot, as described in more detail in U.S. Pat. No. 4,314,833.
The essential difference between the PCVD method and the MCVD method, which is also used for the fabrication of optical fibers by coating the inner wall of a tube, resides in the manner in which the chemical reactions needed for the precipitation of the glass components are activated. Whereas the MCVD method is based on thermal activation using a burner (torch), the PCVD method is based on activation by electron excitation. In the MCVD method, primarily fine particles of glass soot are produced. The uniform deposition of the glass soot in the temperature and gravitational fields is achieved by rotating the tube. To obtain compact layers, the deposited material must subsequently be sintered.
In the PCVD method, in contrast, there is no formation of fine soot-like particles. The gaseous reaction products are in molecular form and reach the inner wall of the tube, and condense there, by relatively fast diffusion. Consequently the deposition is narrowly localized and takes place uniformly over the periphery of the tube. Subsequent sintering is not necessary. As a result, in the PCVD method glass can be precipitated in both directions of the plasma movement. In the MCVD method, glass particles can only be deposited in one direction, namely in the direction of the reactive gas flow.
The economics of the two methods of fabricating optical fiber preforms depends essentially on the yields attainable with the specific fabrication processes. Basic parameters in this connection are the reproducibility of the process steps, the chemical reaction yield, the deposition rate and the optical and geometric homogeneity of the deposited material along the length of the preform.
Because of its special reaction and deposition mechanisms the PCVD method can be used to fabricate optically high grade preforms with high reproducibility, with reaction yields of almost 100%, and, compared with other methods, low end taper losses. End tapers are understood to be deposition zones at both ends of the preform where the optical and geometric properties are not sufficiently constant. The length of preform between the end tapers will be referred to as the homogeneous plateau region. Since the transport to the tube wall in the PCVD method is determined by fast, molecular diffusion mechanism, the extension of the deposition in the axial direction of the support tube by nature is small. Homogeneous plateau regions are the result of moving the reaction zones back and forth along the tube at a constant speed. The regions of nonconstant speed necessary for reversing the direction of the plasma at the ends of the preform are chosen to be as small as possible.
In the PCVD method, with deposition rates of about 0.5 g/min and preform lengths of about 70 cm, end taper losses are about 15%, implying an end taper length of about 10 cm. For practical reasons the end taper length is defined as the distance between the locations at which the mass of the deposited glass amounts to 10% and 90% of the maximum value in the plateau region.
Due to the different types of reaction and transport mechanisms in the MCVD process, and in particular due to the formation of glass soot in the homogeneous gas phase reaction, the MCVD process results in extended deposit distributions along the length of the tube and thus requires additional measures to reduce end taper losses.
A method of reducing the taper in fibers produced by the MCVD process, described in U.K. Patent Application No. 2,118,165, consists of moving the torch along the tube with a variable velocity (i.e. nonlinearly) by mechanical means. The nature of the movement depends on the specific deposition function, which in the MCVD method depends in a complex way on all deposition parameters. This function must be established experimentally by an iteration procedure for the existing special deposition conditions. In all cases, a special nonlinear form of movement along the entire deposition length is required.
Other possible ways of reducing taper, for example flow modifications or linear mechanical ramping consisting of slowing down the torch velocity at the entrance of the tube, were discussed but were found to be unfeasible or their effect was not sufficiently evaluated. The reason for this, and consequently for the above-described special measures for reducing the taper, is attributable to the following specific properties of MCVD method:
1. In the MCVD method, the particle size distribution and the behavior upon the incorporation of SiO.sub.2 and dopants depend, due to the glass soot formation, in an extraordinarily complex manner on all of the process parameters. Consequently, the deposition function can be experimentally determined but not quantitatively calculated from theory.
2. As a consequent of the glass soot formation, which is connected with SiO.sub.2 deposition yields under 100%, the local deposition profiles in the MCVD method are larger than the lengths of travel between reversal points. That is, the soot particles deposit on an even larger volume from the end of the tube. Consequently, to achieve reasonably constant layer thicknesses, the torch must be moved nonlinearly over the entire length of travel. A homogeneous plateau region in the real sense does not exist in the MCVD method.
3. A variation of other process parameters, such as the torch velocity, to reduce the taper has the effect, again due to the complex behavior of dopant incorporation, yield and soot particle size, of immediately, and simultaneously, changing the deposition profile or deposition function, the dopant incorporation, and the yields. Such variations are therefore difficult to control in the MCVD process and cannot be implemented without negative effects on the optical quality and homogeneity.
As may be concluded from the foregoing description of the state of the art, the PCVD method offers, as far as end taper losses are concerned, advantages which are intrinsic in the process. In the PCVD method it is unnecessary to use the elaborate and problem-causing measures adopted in the MCVD method to reduce end taper and to achieve homogeneous plateau regions in the preforms.
Nevertheless, even in the PCVD method it is desirable to achieve a further reduction of end taper losses so as to increase the process yields. If the deposition rates are increased to above 0.5 g/min, it is particularly desirable to ensure that, even under the process conditions then prevailing, the relative end taper losses remain sufficiently small (i.e. typically less than 20% of the preform length).